The present invention relates to a power transmission mechanism for transmission of torque between two members.
Transmission shafts for transmission of power (torque or rotation) are used in many machine parts in automobiles and industrial machines. Some shafts are solid and others hollow, these being produced by direct cutting or plastic working of bar or pipe material or, in recent years, by the sintering of powder.
Spline shafts or serrated shafts for transmission of high torque are generally formed by subjected medium carbon steel or low alloy steel (case hardening steel, nitrided steel or the like) to a heat treatment, such as surface hardening process or tempering, for example, carburization hardening, high frequency hardening or nitriding, so as to increase the shaft strength in consideration of plastic workability, machinability and cost, it being only after such treatment that these shafts are put to use. Further, recently, use has been made of non-refine steel to dispense with refining, or a material subjected to high alloying or high purifying (reduction of inclusions, reduction of P, i.e., phosphorus, etc.) to increase strength, or a material subjected to shot peening to increase fatigue strength.
FIG. 5 shows an example of a machine part having said transmission shaft, which is a constant velocity joint used in the drive shaft of an automobile. This constant velocity joint includes a shaft member 11 having an inner ring 12 fitted thereon through splines 13 formed on the outer periphery of said shaft member 11, the torque in the shaft member 11 being transmitted to the inner ring 12 through the groove-and-ridge fit of the splines 13.
In this connection, there are various types as to the shaft of the spline terminal end side (C in the figure) of the shaft member 11xe2x80x94the xe2x80x9cterminal end sidexe2x80x9d means the opposite side when the shaft end surface which, when the shaft member 11 is inserted in the inner ring 12, is the first to fit in the inner ring 12 is taken as the inlet side. FIGS. 6 through 9 show examples thereof, FIG. 6 showing a first type (tentatively referred to as the xe2x80x9ccut-through typexe2x80x9d) in which the spline trough 11a of the spline 13 is directly cut through the outer peripheral surface of the shaft member 11, FIGS. 7 through 9 showing a second type (tentatively referred to as the xe2x80x9ccut-up typexe2x80x9d) in which the spline trough 11a is smoothly diametrically increased until it connects to the outer peripheral surface of the shaft member 11. Different forms of the cut-up type are known: one in which the diametrical increase is effected by an arc with a radius R1 (FIG. 7), one in which the diametrical increase is effected by an arc with a greater radius R2 than in FIG. 7 (R2 greater than R1) (see FIG. 8), and one in which the diametrical increase takes a spherical form with a radius SR (see FIG. 9).
FIG. 10 shows a conventional fit between said shaft member 11 and inner ring 12, wherein a relief region Txe2x80x2 where the inner diameter is increased is defined in the inner ring 12 on the terminal end side of the spline ridge 12b, and the portion of the ridge 12b excluding the relief region Txe2x80x2 is fitted in the portion of the trough 11a excluding the diametrically increased region Sxe2x80x2 of the shaft member 11, it being arranged that such fitting portion Fxe2x80x2 (marked with dots) does not enter the diametrically increased region Sxe2x80x2 of the trough 11a of the shaft member 11.
In recent years, with the global environmental problem being highlighted, it has been required in the automobile industry to tighten emission control and improve fuel efficiency, and as a measure therefor, lightening has been promoted. In automobiles, splines and serrations (hereinafter represented by the term spline shaft) have been used abundantly in such parts as transmissions, differentials, drive shafts, and propeller shafts, it being noted that since the reduction of the weight of the spline shaft contributes much to the lightening of the automobile, there has been a strong need to increase the spline shaft strength, i.e., to increase the strength in two aspect: static strength and fatigue strength.
As for the measures for strengthening and lightening the spline shaft, the aforesaid high alloying or high purifying may be contemplated, but these would not be advantageous from the viewpoint of production cost since they are attended by an increase in the cost of material or a large decrease in workability. Further, shot peening, though effective in improving the fatigue strength, is not observed to provide sufficient merits as to static strength; rather, it leads to high cost.
A spline shaft whose terminal end is increased in diameter in a large arc form (FIG. 8) or in a spherical form (FIG. 9), though improved in static strength as compared with the type shown in FIG. 7, is not observed to provide sufficient merits in increasing the fatigue strength, as can be seen from the test results shown in FIG. 13. Further, since working tools (hob cutters, rolling racks, etc.) have to be newly produced, a cost increase is incurred. On the other hand, the cut-through type shown in FIG. 6 is not suitable for weight reduction measures, since it is inferior to the cut-up type shown in FIG. 7 in both static strength and fatigue strength, as is clear also from the experimental results shown in FIG. 12.
As described above, the conventional measures for weight reduction are confronted with problems in either cost or strength and there has been no measure that has successfully satisfied both of the requirements at one time.
Accordingly, an object of the present invention is to make it possible to achieve improvements in the static strength and fatigue strength of a spline shaft or serrated shaft without incurring an increase in costs.
The boss of the inner ring was fitted on a spline shaft of the type shown in FIG. 7 whose troughs were diametrically increased by an arc (for the specifications of the spline shaft, see FIG. 14), and this assembly was put to torsion test to examine and analyze the fracture mode. As a result, it was found that as shown in FIG. 11, the fracture comprised two main fracture planes A and B: a first fracture plane extending along the bottom of the trough 11a of the shaft member 11 (A: axial fracture plane), and a second fracture plane inclined at an angle of 45xc2x0 with respect to the axis (B: main stress fracture plane). It is believed that the axial fracture plane A is a shear fracture plane due to an axially-acting shearing force and that the main fracture plane B is a tensile fracture plane due to torsional main stress.
Next, the fit position of the boss was axially stepwise shifted and the strength of the spline shaft was measured at each shifted position; the results shown in FIG. 15(A) were obtained. The horizontal axis in this figure represents the position X [mm] at which the boss is fitted, and the left-hand vertical axis represents a ratio Y1 of repetition till the occurrence of fatigue fracture (the load shear stress is set to xc2x1665 MPa [67.8 kgf/mm2] and the right-hand vertical axis represents the rate of increase Y2 [%] in torsional break strength. The X on the horizontal axis, as shown in (B) of the same figure, indicates the distance from the terminal end 11a1 of the trough 11a of the shaft member 11 to the point (xe2x97xaf) where the terminal end outline 12b1 of the ridge 12b of the boss 12 crosses the outer periphery level L of the shaft member 11. Measurements were conducted at positions X=a, b, . . . e, the repetition ratio Y1 and rate of increase Y2 being determined with the position a (X=6 mm) used as a reference (Y1=1, Y2=0). Further, (2), (4), (6), (10), and (12) in FIG. 15(A) indicate axial shear crack lengths [mm].
It is seen from FIG. 15(A) that the more the fit position of the boss is shifted toward the terminal end (left-hand side of the figure) of the shaft member 11, the more the axial shear fracture plane (axial shear crack) decreases, thus increasing the strength. The reason, as it is believed, is that the portion of the spline (non-fit portion) which is not fitted to the boss during the torsion test is locally twisted and that if the length of the non-fit portion is decreased, the local twist decreases and so does the shear stress acting on the trough of the shaft in the non-fit portion.
It is seen from FIG. 15(A) that the static strength and fatigue strength sharply increase after the boss has reached a particular fit-position which is immediately in front of the terminal end of the spline shaft. The critical position where the fatigue strength sharply increases lies in a region between the points b and c in (B) of the same figure, said region roughly coincides with the position where the terminal end rise portion 12b1 of the ridge 12b of the boss starts to cross the diametrically increased region Sxe2x80x2 of the trough 11a of the shaft member 11 (the position where the rise portion 12b1 of the ridge 12b starts to fit in the trough 11a in the diametrically increased region Sxe2x80x2).
It is believed that the reason is that in addition to the decrease in the shear stress in the tooth bottom of the non-fit region described above, in the diametrically increased region Sxe2x80x2 the trough 11a is diametrically increased, resulting in the tooth bottom being diametrically increased, so that the stress in this portion is decreased.
The present invention, which is based on the view described above, is intended to provide a power transmission mechanism wherein a shaft member and an outer peripheral member disposed on the outer periphery of said shaft member are connected for mutual torque transmission by a fit between the teeth of the shaft member and the teeth of the outer peripheral member and the troughs of the teeth of the shaft member are increased in diameter at least at one axial end, said power transmission mechanism being characterized in that in the diametrically increased regions of said troughs, there are fit regions for the teeth of the shaft member and the teeth of the outer peripheral member. In this case, the shaft member and the outer peripheral member are connected by splines or serrations.
In the diametrically increased region of said trough, if the trough in the tooth of the shaft member is kept in contact with the ridge of the tooth of the outer peripheral member, then the fit region is provided with a sufficient area to greatly increase the strength. In this case, if an arcuate chamfer is provided on the ridge of the tooth of the outer peripheral member in contact with the trough in the tooth of the shaft member, it is possible to bring the two in surface contact with each other in the diametrically increased region, and the increased area of the fit region provides a further increase in strength.
Further, the ridges of the teeth of the outer peripheral member may be brought into contact with the large diameter end (the terminal end) of the diametrically increased region of said trough, thereby providing a sufficient area for the fit region, achieving a large increase in the shaft strength. In this case, it is preferable to provide a restricting means for preventing the outer peripheral member from moving toward the other axial end to serve as a means for preventing the rattling of the outer peripheral member.
Such restricting means may be composed of a forcing means for forcing the outer peripheral member toward one axial end or a pressing means by which the teeth of the shaft member and the teeth of the outer peripheral member are circumferentially pressed against each other.
As described above, according to the invention, it is possible to greatly increase the static strength and fatigue strength of a spline shaft or serrated shaft. Furthermore, there are no such drawbacks as decreased workability and increase in costs, as in the case of using high alloy steel or highly purified steel, nor is there a marked increase in facility introducing cost as in the case of shot peening. Thus, reduction in the cost and weight of the spline shaft is made possible and in the present invention it is possible to achieve, for example, a 12% reduction in weight since a 19% increase in strength can be achieved.